The open-source Mesa/X.Org developers have been working on LLVMpipe, a Gallium3D driver that accelerates OpenGL and other state trackers on the CPU rather than any GPU driver to provide a better software rasterizer via LLVM optimizations. Unfortunately, it's still slow and can barely keep up with games.

There is no mention about image quality of LLVMpipe anywhere, but I assume it produces quite nearly the same output as a real 3D accelerator does.

Unless there you are using some unsupported feature, or there is a bug, the image should be pixel perfect. Rendering is a very precise thing - there is really no room for "optimization" at the expense of quality.

Also, I suspect that LLVMpipe is more complete than any of the real Gallium drivers - programming the CPU is a lot easier than a GPU... (and I know because I've tried it).